In the comments for this question, I was told that the close-and-reopen process is how the PSE community finds consensus on what is on-topic and what is off-topic. Is this correct?
Voting is how the community comes to consensus on most things on the StackExchange network. Is the question good? Up-votes will determine that. Is the question bad? Down-votes. Does the question fail to meet on-topic criteria? Close votes. Has the question been updated to be on-topic? Reopen votes, etc.
There are many discussions about what constitutes on-topic in Meta and in chat, but ultimately "community consensus" can only be measured by how the community votes.
Edited to add:
I want to mention that sometimes "community consensus" on certain topics does not reflect the written guidelines. When that happens, and it has happened several times on PSE, debates take place, and either the guidelines are changed or consensus is swayed. In either case, how the community votes is still the only way to gauge community consensus on interpretations of the guidelines.